I use two 128 SSD Crucials. One reason is i keep raw images off the OS drive and if I have a failure my raw files are separate from the OS. Money is not really the reason. Also I can have backup with my raws as well files on both drives until I get home to my Drobo to store.
BTW not sure about the warranty could care less myself ( gear is heavily insured against any loss) but it is pretty easy to remove the optical and install the bracket for the second drive. You do have to make at least one unplugging of a connector that maybe sensitive to getting screwed. But the connection for the drive itself is easy peasy. Basically three screws for the bracket to MBP and two screws for the SSD drive itself into the bracket maybe three I forget. Install SSD outside the box into the bracket than make the connection and screw it into the MBP. Make sure you remember location of the MBP screws as you take the back off . I believe it is one or two screws towards the back that are shorter than the others
Also keep my I tunes library on the second drive to give the OS plenty of space to roam. I store nothing on the OS drive pretty much
Why not a single 256 GB SSD?